I bought these, the dt-880 (600ohm) and the hd-650.
And I'll quickly write why I picked the 990's over the others.
The dt-880's were amazing, so comfortable, perfect in my opinion.
While the 990's are really similar, they are tighter on the ears, and I don't like that at all, but they are still quite comfortable.
And the hd-650? They clamp like crazy, I couldn't wear these for over 10 minutes, really uncomfortable.
Now this category is objective af, you have been warned.
I used the LP-phone-amp-g103 to drive all of these headphones so there shouldn't be any issues with the amp.
With that out of the way, lets start with the headphones that really disappointed me.
The 880's had a really lackluster low end, you could say non existing, and with basically no bass it was just a clear no for me and I didn't bother any further with them.
The hd-650 sounded like the 990's if someone put an equalizer on them that turned down every frequency past 3khz down 3-6db.
They sounded muddy in my opinion, especially compared to the dt-990's.
With that being said, I love the by some considered sharp highs of the dt-990's, everything sounds crystal clear, not "muddy" or "washed out". I listen to a lot of vocals so that's a must for me. But as I said, it ain't for everybody since some might say the highs are over saturated, and they are, but it sounds nice. It's something I'm looking for in headphones, are you looking for that? If not steer away from these headphones. Cons: they might be too sharp for some and too tiring if used in long sessions. Other cons are the fact that you can really hear if a song is mixed/mastered like shit, you can hear everything. I'm not gonna say sh*t like :"you suddenly hear instruments that you previously didn't hear" if you suddenly heard an instrument that you didn't hear before on decent headphones that's on you. But you definitely hear if someone didn't bother cleaning up the samples he used in his song, and you might need different headphones or speakers to enioy songs that are badly mixed.
price/performance: 10/10, I mean ¤140 for these is a steal, you don't get better audio quality for that price, but you probably gonna need a headphone amp as for almost all 250+ ohm headphones.